An electric double layer capacitor includes an element including a positive electrode, a negative electrode, and a separator interposed between the positive and negative electrodes. The element is impregnated with an electrolytic solution prepared by dissolving an electrolyte in a solvent. Then, the element and the electrolytic solution are accommodated in a metallic case, and an opening portion of the case is sealed with a rubber sealing body.
As an electrolytic solution used for a conventional electric double capacitor, PTL 1 discloses an electrolytic solution obtained by dissolving triethyl methyl ammonium tetrafluoroborate (TEMA.BF4) in an organic solvent including propylene carbonate. This electrolytic solution has excellent electric conductivity.
Furthermore, PTL 2 discloses an electrolytic solution obtained by dissolving spiro-(1,1′)-bipyrrolidinium tetrafluoroborate (referred to as SBP.BF4) in an organic solvent including sulfolane and chain sulfone. Use of this electrolytic solution makes it possible to produce an electric double capacitor having high heat resistance, durability and excellent withstand voltage. Moreover, deposition of chain sulfone at a low temperature can be prevented.